This invention relates to a long-lasting phosphor and, more particularly, to a novel long-lasting phosphor with improved phosphorescence brightness and lifetime and with various emission wavelengths having a chemical composition RO.a(Al.sub.l-x Ga.sub.x).sub.2 O.sub.3.bM.sub.m O.sub.n.cB.sub.2 O.sub.3.dEu.sup.2+ (where R is at least one selected from the group consisting of alkaline-earth metals and Zn, and M is at least one selected from the group consisting of Y, Sc and Si).
Fluorescence is a phenomenon that a material emits visible ray when excited by an excitation source applied from outside. A fluorescent lamp, a discharge tube and a cathode ray tube (CRT) emit fluorescence. A material which emits fluorescence is called a phosphor. When light emitted by a phosphor lasts after stoppage of excitation for duration of time sufficient for the light to be perceived by the eye, i.e., about 0.1 second or longer, the light is called phosphorescence. A phosphor which has a long persistent phosphorescence is called a long-lasting phosphor or a light storage phosphor. As long-lasting phosphors, known in the art are sulfides and oxides. The ZnS:Cu sulfide long-lasting phosphors have been practically used for several decades but they are disadvantageous in that their after-glow lasts for a relatively short duration of time, i.e., about three hours at the longest. Further, this type of phosphor has a fatal defect in that a decomposition reaction of ZnS+H.sub.2 O.fwdarw.Zn+H.sub.2 S takes place under the coexistence of ultraviolet ray contained in the sunlight and moisture contained in the air, causing the phosphor to become black in color and, as a result, the after-glow characteristics significantly deteriorate within a relatively short period of time. For this reason, this type of phosphor has only limited applications such as a luminous watch and a night-time display of a location in a house.
On the other hand, reports have been made about oxide type long-lasting phosphors of m(Sr.sub.1-x Eu.sub.x)O.nAl.sub.2 O.sub.3.yB.sub.2 O.sub.3 (Chinese Patent CN1053807A) and 4(Sr, Eu)O.7Al.sub.2 O.sub.3 (Chinese journal of luminescence, 12 (1991) 144.). Phosphorescence brightness and lifetime of these phosphors are still insufficient for using them for practical applications.
Since, however, these oxides have an excellent chemical durability and light resistance property, if the phosphorescent properties can be improved largely, they will be expected to have wide applications such as signs for preventing disasters, signs for indicating locations for keeping away from danger and decorations in addition to the existing applications for luminous watches and night time display in a house etc.
It is, therefore, an object of the invention to provide a novel Eu.sup.2+ -activated alkaline-earth metal aluminate long-lasting phosphor having improved phosphorescence brightness and lifetime.